The present invention concerns a pressure sensor unit for use preferably in under water plants for transport and processing of hydrocarbon streams, particularly in connection with “Christmas tree devices” on the sea floor. Pressure measurements are necessary on many different points in such plants. The pressure sensor unit according to the invention is still not limited to such use, but can in general be used where a pipe or chamber containing a process fluid, having a hole or an opening through a wall where a pressure sensor unit can be introduced and connected imperviously to the outside of the wall, and such that an inner end of the sensor unit meets the process fluid environment
In the attached FIG. 1 it is shown a general example of mounting of a general pressure sensor unit 1 through a pipe wall 14, for measurement of pressure in a process fluid 16. It is predrilled a hole 25 through the pipe wall 14, and in the case shown it is in addition arranged a sleeve 15 with receiving flange outermost. The pressure sensor unit 1 is made of two major parts, a flange part 7 and a shaft part 2, such that the shaft part 2 is adapted to the hole 25 in length and diameter, however with diameter clearance such that the shaft part may be introduced in and out without problems. It is however important that the flange part 17 fits tightly against its fitting (the receiving flange of the sleeve), because it may occur great pressure differences between the process fluid environment in the pipe and in the environment on the outside of the pipe (typical sea floor pressures, while the pressure in the pipe reflects the pressure in the hydrocarbon reservoir far below in the underground).
The inner end 6 of the shaft part is shown approximately “flush” with the inner surface of the pipe wall, which is a favourable placement with regard to avoiding unnecessary wear on the shaft part, both from possible mechanical erosion because of the process fluid motion, and from chemical (corroding) effect from the process fluid 16.
The manner of operation is such that the pressure sensor element which is contained in the pressure sensor unit 1, is exposed to the pressure of the process fluid, but via a separating membrane arranged in the inner end 6 of the shaft part and a hydraulic oil between the separating membrane and the pressure sensor element itself. Signal conduction leads from the pressure sensor element and out through the flange part 17, and possibly the flange part contains further electronics for processing of the pressure signal before it is guided out.
Previously known embodiments of such pressure sensor units 1 which are generally shown in FIG. 1, appear from FIGS. 2 and 3.
In FIG. 2 it is shown an embodiment with so called “face mounted sensor element”. Here, a sensor element 4 is mounted completely in the inner end 6 of the shaft part, in a sensor chamber 20 almost in one with the room 21 behind the separating membrane 7, i.e. a room in which the membrane can bend inwards. In this embodiment signal conduction 13 is then guided out of the pressure sensor unit 1 thorough an inner, longitudinal boring in the shaft part 2. The inner end of the shaft part 2 is constituted here concretely by a sensor element holder 3, which in the same manner as the wall in the shaft part 2 is made of thick and during special material, typical “Inconel 625” or higher quality.
Sensor element 4 will here also often be used for measuring temperature, since it finds oneself so close to the process fluid itself. Wire 12 is given as temperature signal wire. The sensor element 4 must then of course be able to withstand high temperatures.
A different principle is used in the embodiment which is shown in FIG. 3, namely with so called “withdrawn sensor element” or “remote seal”. Here, the pressure of the process fluid is transferred via a separating membrane 7 through a hydraulic pipe 10 and back to the flange part 17, where the pressure sensor element 4 is arranged in a sensor element holder 3 with a sensor chamber 20. An advantage with such a construction is that pressure sensor element 4 then only needs to experience a limited temperature because of cooling from the outer environment, which typically is sea water. The process fluid temperature may often lie considerably above what the pressure sensor element 4 normally can withstand.
It will normally be arranged a particular temperature sensor element 9 in a solid block 8 which constitute a membrane base for the separating membrane 7 at the inner end 6 of the shaft part. The membrane base/block 8 is conically formed behind the separating membrane 7 to provide the necessary room 21 for inwards bending of the membrane. Signal conduction 12 extends from the temperature sensor element 9, parallel with the hydraulic pipe 10, within a longitudinal boring in the shaft part 2. The wall in the shaft part 2 is for that matter correspondingly thick and solid as in the embodiment shown in FIG. 2, and is made of corresponding high quality material. The block/membrane 8 is made of the same sort material.
The hydraulic pipe 10 which transfers the pressure of the process fluid to the sensor chamber 20 and up into the flange part 17, goes through the inner boring in the shaft part, which in principle has the pressure of the outer environment (lower), and the hydraulic pipe 10 must therefore be of solid type to withstand the pressure difference. The pressure difference must also be taken up/endured by the sensor element holder 3 and a pressure port part 5 which the sensor element holder and the hydraulic pipe is attached to.
Previously prior art in this area with pressure sensor units, is represented moreover by what is known from the publications WO 2004/097361, WO 02/40957, EP 1128172 and US 2006/0201265. The first mentioned publication, WO 2004/097361, shows a pressure sensor unit for exterior mounting, i.e. with a shaft part which exists on the outside of the wall which delimits the pipe or the chamber with process fluid, and then with a flange part existing inside of the shaft part, i.e. the opposite of the technique which is described in FIGS. 1, 2 and 3 and on which the present invention is based. The publication shows however a centrally arranged hydraulic pipe which transfers the pressure of the process fluid from a separating membrane through a sensor chamber with a pressure sensor element, and which exhibits temperature measurements.
From WO 02/40957 it is know a pressure sensor unit with a somewhat different construction. Here it is also found a separating membrane and a hydraulic pipe which transfers a process fluid pressure to a pressure sensor element. Besides, there seems to exist an arrangement to equalize pressure in the internal hydraulic oil. The pressure sensor unit does not have a flange part and a shaft part arranged as in the closest prior art which is shown in FIGS. 2 and 3.
EP 1128172 also exhibits a pressure sensor unit with a separating membrane and hydraulic pipe to an inner chamber with pressure sensor element, but neither this pressure sensor unit has a design adapted to the use which is relevant for the present invention.
US 2006/0201265 shows some pressure sensor constructions, among these one with a flange part and a shaft part mainly corresponding to the construction which is of interest in the present invention. But the shaft part does not comprise any separating membrane and interior hydraulic pipe. The publication does actually concern sterilizable bushings for use with measurement devices.
A problem with the previously known sensor units of the type shown in FIGS. 2 and 3, is that they are expensive. The material price is a great part of the production cost. The customers demand materials which are extremely resistance against corrosion. Often it is specified (as mentioned earlier) “Inconel 625” or higher quality. Such materials have in themselves a high price, and are also expensive to machine.
To achieve lower consumption of high cost material the present invention therefore suggests reducing the thickness of the shaft part wall. It is possible to go down in wall thickness if one let process fluid into the inner room of the shaft part. Therefore, it is provided according to the present invention a pressure sensor unit as precisely defined in the appended patent claim 1. The special about the pressure sensor unit according to the invention, is that the shaft part inner end is provided with perforations to let process fluid into the inner room of the shaft part, and that the separating membrane is arranged on a block suspended in the inner room of the shaft part immediately behind the perforations.
Preferable and favourable embodiments of the pressure sensor unit according to the invention, appear from the dependent patent claims 2-5.